1. Field of the Invention
The present invention relates to a motor-driven power steering system which employs an electric motor for generating assistive steering power that is applied to a steering mechanism to reduce a steering force to be manually applied, and a to method of controlling such a motor-driven power steering system more particularly the present invention relates to a motor-driven power steering system and a method of controlling the same for the temperature-compensated control of the assistive steering power applied to the steering mechanism based on a change in the temperature within or of the power steering system.
2. Description of the Relevant Art
Conventional motor-driven power steering systems for use on automotive vehicles reduce the manually applied steering force to a steering wheel by transmitting the rotative power of an electric motor through a speed reducer, such as a gear mechanism or a ball-screw mechanism, to a steering mechanism.
Various characteristics of a motor-driven power steering system vary dependent on the temperature of or within the system. The temperature of or within the motor-driven power steering system may varied vary according to the ambient temperature, especially when the system is used in colder environments, or the electric motor of the system, which itself is a heat source in the system.
One of the various characteristics that tends to vary, as the temperature of or within the power steering system varies, is the output torque of the motor which varies upon an increase in the temperature brought about by the motor. An excessive temperature rise might cause damage to the electric insulation of the motor, resulting in a motor burnout. One solution to this problem is disclosed in Japanese Laid-Open Patent Publication No. 60-35663. In the disclosed motor-driven power steering system, the current flowing through the motor is detected and monitored for any increase in the motor temperature. Before the motor temperature rises beyond a prescribed level, the energizing current supplied to the motor is lowered to reduce the amount of heat produced by the motor, thus protecting the power steering system.
The steering mechanism of a motor-driven power steering system has many frictional elements, such as bearings, which are provided with a lubricating material such as grease for smooth operation, long service life, and rust prevention. In the control of the electric motor of the power steering system, the steering feeling, given to the driver by the steering mechanism, is prevented from being lowered due to the frictional elements for the following reason. One torque control signal, among may control signals used for controlling the assistive power from the motor, is divided into and composed of a road load component and a friction component. The friction component of the torque control signal can be controlled to control the motor for overcoming the friction of the steering mechanism, or can be adjusted according to the driver's preference, thus improving the steering feeling.
In such a motor-driven power steering system, the viscosity of the lubricating material varies as the temperature changes. More specifically, in general, the viscosity of the lubricating material is high in a low-temperature range and low in a high-temperature range. When the automobile runs at a low temperature, the viscosity of the lubricating material is increased and the friction component of the torque control signal becomes insufficient, making the steering wheel feel heavy when it starts turning. Conversely, when the automobile runs at a high temperature, the viscosity of the lubricating material is increased and the friction component of the torque control signal becomes excessive, with the result that the reaction of the steering wheel as felt by the driver is undesirably low.